Y

This is like an outer product of Y and d/dX; however, the analogy is not perfect because the gradient operator appears on the right in the matrix product sense and on the left in the operator sense.

C.9 Vectors in Three Dimensions

In this section, we only consider vectors with three real components. For three-component vectors, three products are defined: the dot product, the outer product, and the cross product. The cross product, or vector product, is a vector defined by rU2V,-U,V2